Tubular strings have been sealed in bores with cement. The setting cement can shrink and pull away from the tubular on either side of an annular space or it can pull away from a borehole wall in an open hole cementing application. There can be other causes too such as incomplete mud cake removal or incomplete drilling fluid removal prior to cementing, subsidence and compaction. Cracks can develop later on due to tectonic activities as well. The present invention focuses on gas migration through the set cement as opposed to mitigation of cracks or openings developed after the cement is set. Gas migration through cement can be a dangerous situation and is one of the discussed causes of the Deepwater Horizon accident in the Gulf of Mexico.
Early efforts to counteract gas migration in cement dealt with methods of delivering the cement or the addition of additives to the cement as illustrated by U.S. Pat. Nos. 5,327,969; 5,503,227; 5,199,489; 6,936,574; 7,060,129 and 7,373,981.
In a wholly unrelated field of artificial hip joints shape memory structures were used to retain fixation cement for the hip joint as described in U.S. Pat. No. 6,280,477.
Other applications have involved packers in the annular space that leave channels for cement and use a variety of biasing devices to get the seal material of the packer against the borehole wall. In U.S. Publication 2010/0126735 FIGS. 2 and 3 a base pipe 56 has support members 54 that leave gaps in the annular space 38 for cement to pass. In the FIG. 2B embodiment the member 54 is a shape memory material designed to apply an incremental force to the swelling member 42 off of the tubular 56 to push against the formation 36. Even as to the borehole wall at 36 there are shortcomings of this design in preventing gas migration along the borehole wall. The swelling material can be damaged during run in to the point of openings developing in the swelling layer. The cement in the annular space can still pull away from the seal 42 even if all else functions as planned if the cement experiences shrinkage that causes it to pull away not only from the seal 42 but also from the tubular string 56.
Another attempt at dealing with cement gas migration was an effort by Halliburton to use rubber sleeves on the tubular exterior so that the sleeves are in the annular space. The idea was to pump the cement into the annulus before the rubber rings swelled to hopefully span the annulus with the hope that gas migration at the tubular could be stopped with a bonded seal of the rubber and that the sleeve would push the cement away as it swelled to the borehole wall before the cement set up. The problem with the design is that the swelling process was so slow that the cement would set ahead of the swelling sleeve so that the outer diameter of the sleeve would never reach the borehole wall and the same issues of gas migrations would still be there as the cement got to the borehole wall and the sleeve outer diameter and shrank from both on setting up, leaving open passages at both locations for gas migration.
Multistable structural members are described in U.S. Publication 2009/0186196.
The present invention addresses the issue of gas migration in a new way. It employs shape memory material structures that are secured to the tubular at one end and that when reverting to an original shape, span the annular space by displacing the cement that has yet to set until contact with the open hole or wellbore wall is made that puts the radiating elements of the structure under a compressive load to seal or at least minimize gas migration between zones through the cement. Optionally, the shape memory or bistable structures can be covered in whole or in part with a swelling material. Those and other features of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings with an understanding that the full scope of the invention is determined from the appended claims.